Heat transfer enhancement using CO(2) in a natural circulation loop

The natural circulation loop (NCL) is a highly reliable and noise-free heat transfer device due to the absence of moving components. Working fluid used in the natural circulation loop plays an important role in enhancing the heat transfer capability of the loop. This experimental study investigates the subcritical and supercritical heat transfer performance of a natural circulation loop (NCL) with CO(2) as the working fluid. Operating pressures and temperatures are varied in such a way that the loop fluid should remain in the specified state (subcooled liquid, two-phase, superheated vapor, supercritical). Water and methanol are used as external fluids in cold and hot heat exchangers for temperatures above zero and below zero (in °C) respectively, depending on operating temperature. For loop fluids, the performance of CO(2) is compared with water for above zero and with brine solution for the subzero case. Further, the impact of loop operating pressure (35–90 bar) on the performance of the system is also studied. For hot heat exchanger inlet temperature (5 to 70 °C) and cold heat exchanger inlet temperature (−18 to 32 °C), it was observed that the maximum heat transfer rates in the case of subcritical vapor, subcritical liquid, two-phase and supercritical CO(2) based systems are 400%, 500%, 900%, and 800% higher than the water/brine-based system respectively.